Separate trainee maintainer and reviewer in reviewer roulette

This code that counts trainee maintainers as reviewers may cause confusion because it hides trainee maintainer from the weighted pick. It has caused gitlab-com/www-gitlab-com#9119 where trainee maintainers are also adding reviewer tag.

https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/danger/teammate.rb#L35-39

      # Traintainers also count as reviewers
      def reviewer?(project, category, labels)
        has_capability?(project, category, :reviewer, labels) ||
          traintainer?(project, category, labels)
      end

The problem is that when someone sees the following line, it's not obvious that a trainee maintainer is in both reviewers and traintainers.

        weighted_reviewers = reviewers + hungry_reviewers + traintainers + traintainers